Silicon dendritic web crystals are long, thin ribbons of single crystalline material of high structural quality which are grown in the (111) orientation. The current impetus for developing silicon dendritic-web is its application to the production of low-cost, highly efficient solar cells for direct conversion of sunlight to electrical energy. The thin ribbon form of the crystal requires little additional processing prior to device fabrication, in contrast to wafer substrates from the more traditional Czochralski crystal which must be sliced, lapped and polished prior to use, a costly process even though large volume economies are practiced. Additionally, the rectangular shape of the silicon ribbon leads to efficient packing of individual cells into large modules and arrays of solar cells.
An important aspect of successful dendritic web crystal growth is the configuration of the shields and the lid used to cover the hot silicon melt. In the past, lids having a so-called "dog-bone" slot have been used with some success. Such lids also typically include a pair of outboard holes spaced a predetermined distance from the ends of the slot. These outboard holes assist in maintaining the growing dendritic web crystal at a constant width. Also included in conventional lid configurations are a pair of holes at one end of the lid. These holes are used as entry and exit holes for a laser beam used as a detector, which measures the level of the silicon melt. Another feature typically used in lid configurations is a hole through which silicon pellets are dropped for melt replenishment. Such holes are typically located at the end of the lid opposite from the laser beam holes.
Conventional lid configurations for web growth have produced a maximum of approximately 40,000 square centimeters per week per furnace. A negative characteristic of web grown from a conventional configuration is a pronounced thinning of the web immediately adjacent to each dendrite. The output of this configuration is further limited by crystalline degradation which originates in the thinned areas. The result is often termination of the crystals, thus limiting output, since termination of growth requires nonproductive and less-productive time to restart and widen web crystals.
Another problem associated with conventional lid designs is that the laser beam holes and the replenishment hole are asymmetrical with respect to each other, and thus upset somewhat the delicate thermal balance sought to be achieved by creating lids with symmetrical geometries.